* refactoring config

* Update by comments

* refactoring monitor code, support prometheus
* remove vendor

token is the current token comparison, and oidc generates oidc token using client-credentials flow. in addition - add ping verification using the same method

This option allows users to specify where they want assets to be loaded
from, like the "assets_dir" option that already exists for frps. This
allows library users to use the admin panel without having to bundle
assets with statik.

The ServerCommonConf is now passed around as an argument instead of
being shared between components as a global variable. This allows for
more natural interaction with server.Session as a library and allows for
multiple servers to co-exist within the same process.

Related: #1387